Controlling subsurface pressures while drilling with oil base muds

ABSTRACT

The water percents by volume of input and output circulating oil muds are measured and compared during well drilling operations. When the water percent measurements are made at the surface, the output measurements lag the input measurements so that the same portions of drilling mud are compared. The measurements are made continually (at frequent intervals) or continuously. Accurate measurement of the liquid volume of the water phase emulsified in the oil mud, positive or negative differentials of in and out water percents and overall changes in water percents will determine whether the formation pore pressure is exceeding the hydrostatic pressure imposed by the drilling mud. When formation pressure exceeds the hydrostatic pressure, water from the wet subsurface formations being penetrated enters the well bore and is imbibed (mixed and probably emulsified) in the oil mud. Control of the well is maintained by adjusting drilling mud weight.

Thompson et al.

CONTROLLING SUBSURFACE PRESSURES WHILE DRILLING WITH OIL BASE MUDSFiled:

Inventors: Gene D. Thompson, Louis A.

Carlton, both of New Orleans, La.

Assignee: Esao Production Research Company, Houston, Tex.

Oct. 28, 1971 Appl. No.: 193,322

Primary Examiner-Marvin A. Champion Assistant Examiner-Richard E.Favreau Attorney-Thomas B. McCulloch et al.

[ 57] ABSTRACT The water percents by volume of input and outputcirculating oil muds are measured and compared during well drillingoperations. When the water percent measurements are made at the surface,the output measurements lag the input measurements so that the sameportions of drilling mud are compared. The measurements are madecontinually (at frequent intervals) or continuously. Accuratemeasurement of the liquid volume of the water phase emulsif ed in theoil mud, positive or negative differentials of in and out water percentsand overall changes in water percents will determine whether theformation pore pressure is exceeding the hydrostatic pressure imposed bythe drilling mud. When formation pressure exceeds the hydrostaticpressure, water from the wet subsurface formations being penetratedenters the well bore and is imbibed (mixed and probably emulsified) inthe oil mud. Control of the well is maintained by adjusting drilling mudweight.

.17 Claims, 1 Drawing Figure our PUT Aug. 7, 1973 PATENFEU 7W5 3.750.766

INPUT OUTPUT 1 l k)\\\\\\\\\ l GENE D. THOMPSON, LOUIS A.CARLTON,

5 INVENTORS. n

Z Y BY JOHN S. SCHNEIDER ATTORNEY.

CONTROLLING SUBSURFACE PRESSURES WHILE DRILLING WITH OIL BASE MUDSBACKGROUND OF THE INVENTION Early detection and evaluation of abnormallypressured formations minimizes or avoids potential or actual blowouts.When abnormally pressurized formations are detected, drilling mud weightis increased to balance the increased pressure of the abnormallypressured formations. Many techniques, such as measurement of shaledensities, formation conductivity, formation resistivity, formationacoustic travel times, drilling rate, mud gas and chloride contents ofdrilling mud, have been used heretofore to detect and evaluate formationpore pressures.

In U. S. Pat. application Ser. No. 145,833 filed May 2l, 1971, by LouisA. Carlton et al. entitled DETER- MINATION F SUBSURFACE FORMATION PRES-SURES, a technique is described and claimed in which small subtleincreases in salinity of the drilling mud are measured. Both the inputand output drilling mud salinities are measured at frequent intervals orcontinuously and when compared trends and changes in the salinity of thedrilling mud are readily recognized. An increasing or fluctuating trendin salinity of the drilling mud indicates a flow of salt water fromsubsurface formations open to the well bore. Such flow is evidence thatformation pore pressure is exceeding the hydrostatic pressure imposed bythe drilling mud column. Increasing the drilling mud weight until theinflux of formation fluids is stopped minimizes the problems associatedwith taking formation fluid kicks.

The technique disclosed and claimed in U. S. Pat. application Ser. No.145,833 is not applicable to oil base or invert oil muds (referred toherein by the term oil muds) because of the oil external phase. Thus,for deep wells which require oil muds there is a need for a technique todetermine subsurface pressures.

During conventional drilling operations, the oil industry has learned todetermine chlorides of the water phase internal emulsion in an oil mudby taking a sample, adding strong emulsion breakers, and thendetermining the parts per million chlorides in the water by conventionaltitration. The water percent by volume in oil muds is determined by aretort process which prohibits direct chloride measurements.Furthermore, the salinity (sodium chloride-NaCl and calcium chloride-CaCl content of the water phase in an oil mud is often varied for thepurpose of shale stability. Consequently, this batch" process doe notpermit continuous salinity measurements as conducted in theaforementioned U. S. Pat. application Ser. No. 145,833.

A significant improvement of the present invention over past practicesresides in measuring water percent by volume in the oil mud continuouslyor at frequent intervals to achieve the same end result withapproximately the same accuracy using oil muds as achieved with waterbase muds. The differential (increase or decrease) in water percent ofthe oil mud system before and after circulating down the drill pipe andback up the annulus opposite the new formations just drilled isdetermined. Rather than measuring and comparing the salinity of theinput and output circulating drilling mud while drilling as whendrilling with water base muds, the percent water in an oil mud ismeasured. The output measurement of percent water is properly lagged andcompared with the water percent of the same fluid as it occurred at theinput conditions. Metering or other techniques may be used to measurethe liquid volume of the water phase emulsified in the oil mud, positiveor negative differential and overall changes in water percents determinewhether the fonnation pore pressure is exceeding the hydrostaticpressure imposed by the drilling mud. When the formation pressureexceeds the hydrostatic pressure, water from the wet sub surfaceformations being penetrated will enter the well bore and be imbibed(mixed and probably emulsified) in the oil mud.

SUMMARY OF THE INVENTION.

Briefly, the present invention involves a method for determiningsubsurface formation pressures which comprise measuring and comparingthe water percents of the input and output oil muds being circulatedduring drilling of a well. The output measurement follows the inputmeasurement by a lag time period which corresponds to the time periodrequired for the drilling mud to circulate from the input measurementpoint to the output measurement point. The water percent measurementsare made at frequent intervals or continuously.

BRIEF DESCRIPTION OF THE DRAWINGS The sole FIGURE illustrates one modefor carrying out the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to the figure there isshown a borehole 9 being drilled through subsurface formations indicatedat 10 by a drill bit 11 connected to the lower end of a drill string 12which extends upwardly to the rotary table 13 through casing pipe 15,wellhead 16 and blowout preventer equipment 17. A returns or outputflowline 22 is connected to the annulus surrounding drill pipe 12 abovethe blowout preventer to return the circulating drilling mud to a mudpit 23. An input suction line 24 is located in mud pit 23 and isconnected to a circulating pump 25 which, in turn, is connected to aninput flowline 26 which, as indicated by the dotted lines 27, connectsto the upper end of drill pipe 12. A water percent detector unit 30 isconnected into flowline 22 and a similar detector unit 31 is connectedinto mud pit 23 near the suction end of line 24. These detector unitsare shown connected to suitable recorders 35 and 36 as indicated bylines 32 and 33, respectively.

As illustrative of the operation of the apparatus of the figure,drilling mud is circulated by means of pump 25 from mud pit 23 throughsuction line 24 and discharge line 26 into drill pipe 12. Drilling mudis circulated through drill bit 11 and up the annulus between drill pipe12 and the wall of borehole 9 and the annulus between drill pipe 12 andeasing pipe 15 and through wellhead l6 and blowout preventer l7 and intoreturn line 22 and mud pit 23. Detector units 30 and 31 measureproperties of the drilling muds which enable determination of thepercent water in the drilling muds. The water percent measurements onthe chart of recorder 35 may be adjusted for lag time byphysicallyshifting this chart relative to the chart on recorder 36. Lag time isreadily calculable from the pumping rate, the depth and diameter of thehole being drilled and the size of the drill pipe. Of course, periodicadjustments may be made to increase the lag time as the borehole beingdrilled increases in depth. Lag time adjustment and plotting orrecording of the water percents of the input and output drilling mudsmay be performed automatically. One way to compensate for lag timeautomatically would be to use a circular recorder chart and two pins,one for recording output mud water percent and the other for recordinginput mud water percent. The angle between the recording pins wouldrepresent lag time when a constant pump rate is used. When the pump rateis changed or the lag time is changed, the angle between the pins isadjusted to reflect such change. A differential water percent log may beautomatically plotted by electronically subtracting the input waterpercent measurements from the output water percent measurements prior torecording these measurements. Also, a computer or other memory typedevice might be used to store the input water percent data for the lagtime period prior to substracting the input from the output measurement.

However, plotting of the water percent data may be performed manually.An operator may take readings of the water percent devices at frequentintervals and plot the results. In making these plots the operator wouldtake into account lag time.

Oil mud water percents may be measured downhole, preferably near theformation being drilled, to detect changes in the water percents in theinput and output drilling mud as the formation is being drilled. Thesedata may be transmitted continuously or at frequent intervals tosuitable service recording devices. The procedure is the same as whenmaking surface measurements except no lag time adjustments arenecessary. The devices for measuring water percents of the input andoutput drilling muds would be preferably mounted on the interior andexterior of drill collars or special subs located adjacent the drillbit. Suitably modified conventional well logging tools might be employedin conducting these operations.

The liquid phase of conventional oil muds may vary in water percentsfrom approximately 4 to 40 percent water. Any influx of subsurface waterfrom a porous formation during drilling will be detected by the waterpercent measuring devices and such data will be utilized by theoperators as necessary to increase the mud weight (hydrostatic pressure)to control the well and prevent a blowout. The process is repeatedduring the entire drilling operation in which the mud weight is raisedeach time a differential increase in percent water occurs until the wellreaches total depth.

Measuring the water percents of the input and output drilling muds atfrequent intervals or continuously avoids drastic changes in mud flowproperties by the detection of small formation fluid influxes andadjusting mud weights before enough contamination has entered the mudsystem to thicken the mud substantially. Use of the technique of thisinvention to control mud weights will substantially reduce theoccurrence of lost returns caused by excessive mud density. In additionto the time spent circulating and conditioning mud due to contaminationwill be reduced. When a drilling mud has thickened substantially,operators are frequently forced to stop drilling and thin the mud inorder to avoid lost circulation. The lost time required to condition themud may total several days in deep high pressure wells and maysubstantially increase costs for rig time and mud materials.

Meters for measuring the water percent accurately (0.1 to L percent) maybe of various types. Any of the various kinds of sensing devices capableof sensing the percentage of a component in a fluid mixture and whichproduce a signal having a quantitative response indicative of thatpercentage may be used. For example, the device may be a combinednuclear density gauge and amplifier which provides a sensitive andaccurate measure of the specific gravity and instantaneous percent waterof the oil mud. Another type of sensing device which may be usedincludes a circuit containing a capacitance cell to sense the dielectricconstant of the oil mud passing through that cell. The capacitance cellmay be contacted with the oil mud by means of a probe inserted in theoil mud. The capacitance of the cell is directly proportional to thedielectric constant of the oil mud flowing through the cell and thedielectric constant changes relative to changes in the percent water ofthe oil mud. The percent water by volume in the oil mud may also bedetermined by known techniques such as the retort technique in which thewater is separated from the oil mud by distillation and measured.

Changes and'modifications may be made in the illustrative embodimentsshown and described herein without departing from the scope of theinvention as defined in the appended claims.

Having fully described the nature, objects, operation and advantages ofour invention, we claim:

1. A method for determining subsurface formation pressures during thedrilling of a well comprising the steps of:

measuring the percent water of the input and output oil muds beingcirculated in a well during drilling thereof, said output drilling mudwater percent measurement following said input drilling mud waterpercent measurement by a lag time measurement which corresponds to thetime period required to circulate said drilling mud from the inputmeasurement point to the output measurement point; and

comparing said input and output drilling mud water percent measurements,variations in said input and output water percent measurements providingindications of said formation pressures.

2. A method as recited in claim 1 in which said water percentmeasurements are made continually at frequent intervals.

3. A method as recited in claim 1 in which said water percentmeasurements are made continuously.

4. A method as recited in claim 3 in which said water percentmeasurements are made by measuring electrical properties of saiddrilling muds.

5. A method for detennining subsurface formation pressures andcontrolling a well during the drilling thereof comprising the steps of:

circulating an oil mud down a drill string and up the annulussurrounding the drill string to the earth's surface;

measuring water percents of the input and output oil muds, said outputoil mud water percent measurement following said input oil mud waterpercent measurement by a lag time period which corresponds to the timeperiod required to circulate said oil mud from said input measurementpoint to said output measurement point;

comparing said input oil mud and said output oil mud water percentmeasurements; and

adjusting weight of said oil mud in response to said water percentmeasurements of said drilling muds.

6. A method as recited in claim 5 in which said water percentmeasurements are made continually at frequent intervals.

7. A method as recited in claim 5 in which said water percentmeasurements are made continuously.

8. A method as recited in claim 5 in which said water percentmeasurements are made by measuring electrical properties of saiddrilling muds.

9. A method as recited in claim 5 in which said measured water percentsin the input and output oil muds are separately plotted.

10. A method for determining subsurface formation pressures during thedrilling of a well comprising the steps of:

measuring the water percent of the input drilling muds being circulatedin a well during drilling thereof;

plotting in a first curve the water percent of said input drilling mudsversus depth of said wells; measuring the water percent of the outputdrilling being circulated in said well during drilling thereof; plottingin a second curve the water percent of said output drilling mud versusdepth of said well; and comparing said first and second curves,variations therein providing indications of said formation pressures.

1 l. A method as recited in claim in which the output drilling mudmeasurements are compared with the input drilling measurements by a lagtime period which corresponds to the time period required to circulatesaid drilling mud from the input measurement point to the outputmeasurement point.

12. A method as recited in claim 11 in which said water percentmeasurements plotted in said first and second curves are measured bymeasuring electrical properties of said drilling mud.

13. A method for determining subsurface formation pressures duringthedrilling of a well comprising the steps of:

simultaneously measuring water percents of the input and output oil mudsbeing circulated in a well duringdrilling thereof; and

comparing the input and output oil mud water percent measurements of thesame portions of oil mud, variations in said input and output waterpercent measurements providing indications of said formation pressures.

14. A method as recited in claim 13 in which said water percentmeasurements are made continuously.

15. A method as recited in claim 14 in which said water percentmeasurements are made by measuring electrical properties of said oilmuds.

16. A method as recited in claim 14 in which said water percentmeasurements are made continually at frequent intervals.

17. A method as recited in claim 16 in which said water percentmeasurements are made by measuring electrical properties of said oilmuds.

1. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of: measuring the percent water of the input and output oil muds being circulated in a well during drilling thereof, said output drilling mud water percent measurement following said input drilling mud water percent measurement by a lag time measurement which corresponds to the time period required to circulate said drilling mud from the input measurement point to the output measurement point; and comparing said input and output drilling mud water percent measurements, variations in said input and output water percent measurements providing indications of said formation pressures.
 2. A method as recited in claim 1 in which said water percent measurements are made continually at frequent intervals.
 3. A method as recited in claim 1 in which said water percent measurements are made continuously.
 4. A method as recited in claim 3 in which said water percent measurements are made by measuring electrical properties of said drilling muds.
 5. A method for determining subsurface formation pressures and controlling a well during the drilling thereof comprising the steps of: circulating an oil mud down a drill string and up the annulus surrounding the drill string to the earth''s surface; measuring water percents of the input and output oil muds, said output oil mud water percent measurement following said input oil mud water percent measurement by a lag time period which corresponds to the time period required to circulate said oil mud from said input measurement point to said output measurement point; comparing said input oil mud and said output oil mud water percent measurements; and adjusting weight of said oil mud in response to said water percent measurements of said drilling muds.
 6. A method as recited in claim 5 in which said water percent measurements are made continually at frequent intervals.
 7. A method as recited in claim 5 in which said water percent measurements are made continuously.
 8. A method as recited in claim 5 in which said water percent measurements are made by measuring electrical properties of said drilling muds.
 9. A method as recited in claim 5 in which said measured water percents in the input and output oil muds are separately plotted.
 10. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of: measuring the water percent of the input drilling muds being circulated in a well during drilling thereof; plotting in a first curve the water percent of said input drilling muds versus depth of said wells; measuring the water percent of the output drilling being circulated in said well during drilling thereof; plotting in a second curve the water percent of said output drilling mud versus depth of said well; and comparing said first and second curves, variations therein providing indications of said formation pressures.
 11. A method as recited in claim 10 in which the output drilling mud measurements are compared with the input drilling measurements by a lag time period which corresponds to the time period required to circulate said drilling mud from the input measurement point to the output measurement point.
 12. A method as recited in claim 11 in which said water percent measurements plotted in said first and second curves are measured by measuring electrical properties of said drilling mud.
 13. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of: simultaneously measuring water percents of the input and output oil muds being circulated in a well during drilling thereof; and comparing the input and output oil mud water percent measurements of the same portions of oil mud, variations in said input and output water percent measurements providing indications of said formation pressures.
 14. A method as recited in claim 13 in which said water percent measurements are made continuously.
 15. A method as recited in claim 14 in which said water percent measurements are made by measuring electrical properties of said oil muds.
 16. A method as recited in claim 14 in which said water percent measurements are made continually at frequent intervals.
 17. A method as recited in claim 16 in which said water percent measurements are made by measuring electrical properties of said oil muds. 